util-lib: split out user/group/uid/gid calls into user-util.[ch]
diff --git a/src/tmpfiles/tmpfiles.c b/src/tmpfiles/tmpfiles.c
index 457d721..05c4661 100644
--- a/src/tmpfiles/tmpfiles.c
+++ b/src/tmpfiles/tmpfiles.c
@@ -59,6 +59,7 @@
 #include "specifier.h"
 #include "string-util.h"
 #include "strv.h"
+#include "user-util.h"
 #include "util.h"
 
 /* This reads all files listed in /etc/tmpfiles.d/?*.conf and creates